Create phet-io client guides

[arouinfar]

This sim is on the docket to get republished this quarter and will need client guides.

Client guide work is currently blocked by https://github.com/phetsims/phet-io-client-guides/issues/9 and https://github.com/phetsims/studio/issues/202.

[arouinfar]

The blocking issues have been closed, so I should move forward with the client guides.

@kathy-phet can you forward me the client requests for Gravity Force Lab?

[arouinfar]

The client guides were created in the above commit, but client-requests.md contains only the generic template info and will be edited once I have the specific client requests for this sim.

[arouinfar]

Spoke with @kathy-phet and she said that there weren't specific client requests for this sim, but we decided to include just the generic template items. Unfortunately, those generic items are only applicable to multi-screen sims.

This is a bit of a longshot, but @zepumph is there a way to eliminate/skip over client-requests.md during a build so it doesn't end up on the wrapper index page?

[arouinfar]

Discussed in PhET-iO meeting, and it turns out that the build process is smart enough to skip over non-existent guides. I will delete client-requests.md since there aren't any specific requests for this sim.

[arouinfar]

I deleted client-requests.md in the above commit.

I think the commits in this issue will need to be cherry-picked, so back to @zepumph.